The mammalian neurohypophysial hormones arginine vasopressin (AVP) and oxytocin (OXT) are primarily synthesized in the hypothalamus as large molecular weight prohormones composed of AVP or OXT and their sequence specific carrier proteins, the neurophysins. In most submammalian vertebrates, two different but homologous hormones, arginine vasotocin (AVT) and mesotocin (MTC) represent the primary neurohypophysial peptides and it is assumed that AVT and MTC also are associated with specific neurophysins, although the precursor molecules have not been isolated. It is believed that the transition from AVT and MTC to the mammalian neurohypophysial hormones AVP and OXT represents an important aspect of the evolution of intrauterine fetal development. However, significant evidence has been presented to indicate that AVT also is present in fetal but not adult mammalian pituitary glands of a variety of species. Since the molecular mechanisms involved in the transition from AVT and MTC to AVP and OXT are unclear, the presence of AVT in the mammalian fetus suggest that the MTC and AVT genes might be present in mammals. The presence of AVT in the fetal mammal is of great interest not only from an evolutionary standpoint, but also due to the intriguing possibility that AVT is functionally significant during embryonic and/or fetal development. Previously, we have employed specific antisera for AVT, AVP and OXT combined with high pressure liquid chromatography to identify AVT, AVP and OXT in plasma of fetal lambs and human newborns. In order to confirm AVT synthesis in the fetal mammal, we propose to isolate the respective AVT, AVP and OXT neurophysins, develop specific antisera for each neurophysin, and verify the specificity of each neurophysin assay on the basis of physiological responses to selected stimuli. Subsequently, we will employ our AVT, AVP and OXT radioimmunoassays in combination with the developed neurophysin assays as probes to: 1) identify the sites of AVT, AVP and OXT synthesis in the ovine fetus and 2) to quantify the factors controlling AVT, AVP and OXT synthesis and secretion during development.